The art of fabricating electronic devices based on HTS film structures has evolved rapidly over the past few years. Microwave HTS devices have entered the market (see Witthers, "Superconductor Devices" edited by Ruggiero et al, Academic Press, 1990, p. 227). Integrated circuit HTS devices have been demonstrated in the lab (see Lee et al, Appl. Phys. Lett. V 59, p. 3051, 1991). Further development of HTS electronic devices requires improves integrated circuit fabrication technologies. The principal problems to be solved concern epitaxial insulating buffer layers and the need for reliable patterning processes.
The inventor and others have studied the Si-YBCO intermixed HTS system in some detail (see Ma et al, Appl. Phys. Lett. V 55, pp. 896-898, 1989; and, J. Elec. Mat. V 21, p. 407, 1992). In these studies, silicon ("Si") was introduced into a YBaCuO ("YBCO") HTS film by thermal diffusion through a Si film or Si substrate during the high temperature process used to form the YBCO film. The studies revealed that the chemical reaction between Si and YBCO, and the consequential formation of Si oxide, inhibit the superconducting characteristics of HTS film, causing the affected film portion to acquire an electrical insulating characteristic in place of its former superconducting characteristic. Accordingly, Si can be used to pattern HTS films by locally inhibiting superconductivity in selected portions of the film.
In general, inhibition processes involve the introduction of a reactive impurity (e.g. Si) to remove oxygen ("O") from an oxide superconductor. For example, introduction of Si ions into a YBCO HTS material as described above, breaks down the Cu--O chemical bonds, with the Si itself becoming oxidized to form an insulating oxide compound. The reactive impurity may be any one of a group of materials which are more reactive with oxygen than the element in the oxide superconductor (e.g. Cu, Ba). Elements such as Si, Ti, Al, Mg, Sr, Ni, B, Ce, Ge, Fe, Zr, or Nb; and, compounds such as Si.sub.3 Ni.sub.4, SiF.sub.2 or SiF.sub.3 are suitable reactive impurities. The oxide superconductors include La--Sr--Cu--O, Ca--Sr--Cu--O, Y--Ba--Cu--O, Bi--Sr--Ca--Cu--O, Tl--Ba--Ca--Cu--O, Hg--Ba--Ca--Cu--O, Bi--K--Ba--O, Nd--Ce--Cu--O, etc.
The prior art has evolved a technique for patterning single layer YBCO films (see for example Fork et al, Appl. Phys. Lett., 1990, 57, p. 1137; and, Copetti et al, Appl. Phys. Lett. 61 p. 3041). This technique, referred to as inhibition or reactive patterning, has been used to fabricate various HTS devices, including current controlled switches (see Ma et al, Cryogenics V 30, pp. 1146-1148, 1990); weak-links (see: Meng et al, IEEE Trans. Mag., 1991, 27, p. 3305); SQUIDs (see: Friedl et al, Appl. Phys. Lett., 1992, 60, p. 3048); and, microbolometers (see: Cole, SPIE, 1991, Vol. 1394; and, Parsons et al, Digest of "17th Intl. Conf. on Infrared and Millimeter Waves", Los Angeles, Dec. 1992). HTS microbridges as narrow as 0.13 .mu.m have been made using Si.sub.3 N.sub.4 for Si-YBCO intermixing (see Kern et al J. Vac. Sci. Tech. B, 1991, 9, p. 2875).
A commonly used prior art patterning technique involves removal of material by chemical etching or ion-milling. This leaves a stepped patterned surface, which causes problems if additional layers are to be grown and patterned atop the initially patterned layer. For example, to make a connection between two HTS layers separated by an insulating layer, ion milling has been used to make a channel through the insulating layer. The channel must be made at a small angle relative to the insulating layer surface and the lower HTS layer surface to facilitate growth of a continuous HTS layer. It is very difficult to control the ion milling process to achieve the necessary angle if the initially patterned layer is not flat.
A further disadvantage of the prior art technique is that an inordinately large number of layers must be processed, with each layer being grown epitaxially at high temperatures. For example, an HTS SQUID magnetometer may require as many as 15 epitaxial layers, with 3 HTS layers (see Lee et al, supra). This increases the cost and decreases the yield of device fabrication. Moreover, some contamination of the HTS film during the patterning process is unavoidable. This degrades device performance. If more layers are processed the risk of contamination increases, lowering the performance characteristics of the fabricated device.
This invention provides a method of inhibiting the superconducting characteristics of a selected portion of an HTS film or single crystal by implantation of impurity ions while preserving the crystalline structure of the HTS material and thereby simplifying the patterning of HTS multilayer structures.